1. Field of the Invention
The present invention relates to a radio communication system, the communication system comprising: one master station such as a base station or an access point which transmits information or data without being controlled by another radio station and which is capable of independently setting a channel for use in a radio communication; and a number of slave stations such as terminals or stations whose radio communication channel used and transmission/reception operations (slave mode) are controlled by the master station thereof. More particularly, the present invention relates to a communication method in the case where a master station or a slave station has detected an interference signal during a radio communication performed between the master station and the slave station when a wireless local area network (wireless LAN) is operated.
2. Description of the Related Art
A wireless LAN which has begun to spread from the installation in personal computers (PCs) is being gradually expanded to various other products, such as PC peripheral devices, mobile telephones, game machines, home electric appliances and in-vehicle navigation systems. The wireless LAN is compliant with a standard “IEEE 802.11” defined by the Institute of Electrical & Electronics Engineers in the U.S.
The wireless LAN mainly used at present is compliant with the “IEEE 802.11b/g” standard, and is available at a relatively low cost. However, this wireless LAN uses a 2.4 gigahertz (GHz) band as a radio communication channel, so that if it is installed in many products, expanded into various markets and comes into wide use as described above, a jam is caused in the use of the channel, and there arises a problem of the depletion of frequency resources which have only been assigned up to about the 83.5 megahertz (MHz) band. In particular, the 2.4 GHz band is also released as an industry medical science (ISM) band. This is actually in operation in applications other than the wireless LANs, such as a point of sale (POS) system, an in-warehouse product management system (TAG system), microwave ovens, a telemeter system, a video materials transmission system and cordless telephones. A well-known system operated in the same frequency band is Bluetooth (registered trademark) which has started to rapidly grow in the installation into, for example, mobile telephones to achieve wireless headset (earphone) and handsfree functions. Presently, the number of shipments and the degree of market dominance (the degree of market occupancy) of mobile telephones tend to be on the increase, and there is an urgent need for measures against future depletion of frequency resources.
To meet the above-mentioned needs of the market, a policy to release a new frequency band available for wireless LAN has been continuously executed from around year 2000. For example, a 100 MHz band ranging from 5.15 GHz to 5.25 GHz was released in the year 2000, and a 100 MHz band ranging from 5.25 GHz to 5.35 GHz was released in the year 2005. Then, a 255 MHz band ranging from 5.47 GHz to 5.725 GHz was newly released in the year 2007. As a result, the total frequency band available for wireless LAN in the 5 GHz band has attained 455 MHz. Thus, if a 100 MHz band for the industry science medical (ISM) band (a band of 5.725 GHz to 5.825 GHz) assigned to a 5.8 GHz band is included, a band of 555 MHz is available for wireless LAN. Since a band about 6.6 times as wide as 83.5 MHz on a 2.4 GHz band is available, the problem of the depletion of the frequency resources is expected to be solved.
However, out of the above-mentioned 555 MHz band, a total of 355 MHz including the 5.25 GHz to 5.35 GHz band and the 5.47 GHz to 5.725 GHz band is assigned to radiolocation, e.g., meteorological radar as a primary service. Since the wireless LAN is defined as a secondary service, the wireless LAN using this band is obliged to avoid interference with the primary services. Specifically, it is required that a radar detecting function be mounted in a radio facility operating the wireless LAN and that such a facility obey regulations for the operation of the wireless LAN during radar detection.
Common regulations for the mounting of the radar detecting function and the operation of the wireless LAN during the radar detection are set forth in the statute books of the Radio Law of Japan (radio installation rules), the federal communications commission (FCC) regulations of the U.S., and the European telecommunications standards institute (ETSI) regulations of Europe. In other words, these regulations are defined in accordance with a recommendation (universal standard) made in “world radiocommunication conference (WRC) 2003” held in the international telecommunication union-radiocommunication sector (ITU-R) in year 2003.
In the case of a radio communication system comprising one master station such as a base station or an access point which transmits information or data without being controlled by another radio station and which is capable of independently setting a channel for use in a radio communication, and a number of slave stations such as terminals or stations whose radio communication channel used and transmission/reception operations are controlled by the master station thereof, the master station is obliged to be mounted with the radar detecting function and to inform the slave station of a detection result as well as to control the transmission/reception operations of the slave stations. In addition, the slave stations are exempted from the mounting of the radar detecting function as long as they perform the transmission/reception operations in accordance with the instructions of the master station.
Furthermore, the regulations include operation regulations for the case where radar waves are detected after a communication has been started between the master station and the slave station. The master station is always required to take an evasive action based on these operation regulations in the event of a radar detection (in service monitoring) during the communication. Specifically, when the master station detects radar waves during a communication, the operation in a current radio communication channel used so far is forbidden, and the master station has to move to a new radio communication channel together with all the slave stations. In this case, also specified are the total time of communication (260 msec) between the master station and the slave station starting from radar detection, and the time (10 sec) for the stopping of the operation in the current radio communication channel and the completion of the move to the new radio communication channel, providing insufficient time. In other words, the master station which has detected the radar waves has to inform, within the above-mentioned total communication time (260 msec), all the slave stations belonging thereto of instructions for the terminations or discontinuation of communications currently established and of moves to a new radio communication channel. According to “IEEE 802.11h”, which is a frequency management standard, moves are simultaneously made to the new radio communication channel on the basis of beacon timing issued from the master station.
However, in the case of a radio communication system such as a basic service set (BSS) comprising a master station and all slave stations belonging to this master station, the operational problem is whether all the slave stations can be securely informed of information on the communication restriction (termination or discontinuation) and on the new radio communication channel to move to. In particular, for example, when the number of slave stations is high or when there is a slave station in a sleep mode or when a slave station is away from the master station, it is difficult to ensure that all the slave stations are informed of the evasive action after radar detection.
In “IEEE 802.11h”, which is the frequency management standard, there are regulations concerning logical mechanisms, for example, operation procedures in the event of a radar detection, and communication message configuration therefor. However, no regulations have been set to cover a method of ensuring that the evasive action after the radar detection is reported from the master station to the slave stations in view of mandatory standards (radio laws) defined in various countries and regions.
One of the simplest methods that can be achieved under such various binding conditions is timeout processing in the slave stations carried out by unilateral forced disconnection from the master station. However, the major premise of operating the wireless LAN at the frequency band mandating the radar detection is that “the radio communication channel used and the transmission/reception operations of the slave stations are controlled by the master station”. In other words, if the slave station is allowed to freely run by the unilateral forced disconnection, this can go against the regulations. Moreover, this method is disadvantageous to the slave stations (e.g., users) because the information on the new radio communication channel to move to is not conveyed to the slave stations.
Furthermore, even if the slave stations are controlled by the master station in accordance with such regulations, some characteristic operation method is needed to ensure that the evasive action after the radar detection is reported to all the slave stations within the limited total communication time (260 msec), for which no considerations and regulations have been made so far.
As described above, in order to meet the market needs for the wireless LAN, the new frequency band has been released for the spread of the wireless LAN. However, in the case of a radio communication system comprising a master station and slave stations, when the master station has carried out a radar detection, the mounting and operation of a function conforming to the current mandatory standards (radio installations rules, FCC regulations, ETSI regulations) and the standard (IEEE 802.11h) alone do not make it possible to ensure that the master station can report the evasive action after the radar detection to the slave stations, which can cause inconvenience and disadvantages to the users, such as disconnection during operation.
In addition, as a technique associated with the present invention, there has already been made a proposal in which interference between Bluetooth equipment and wireless LAN equipment operated at the same 2.4 GHz band is detected, and a channel quality is estimated using a packet error rate to avoid the interference (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-211242).